Films are provided to users in the form of a film roll obtained by winding a film around a cylindrical core. The following defects may occur in a film roll.
(1) Wrinkling: The film wound around the core has waves.
(2) Slippage: The film wound around the core shifts in a width direction of the core.
(3) Creep: In a case where thickness of the film wound around the core varies in a width direction of the film, the film becomes deformed. Once such a film is wound off from the core, it exhibits curving or retains deformation caused by compression and tension.
Note that wrinkling may also be called a “star defect”, and slippage may also be called “telescoping”.
As an attempt to prevent film roll defects, Patent Literature 2 and Non-Patent Literature 2, in particular, examine determining a winding tension, applied to a film, in accordance with (a) stress acting on film in the film roll (this stress hereinafter referred to as “internal stress”) and (b) frictional force between layers of the film.
Non-Patent Literature 5 analyzes internal stress by focusing on strain residing in the film in a film roll (residual strain). Note, however, that the discussions in non-Patent Literature 5 utilize a simplistic assumption with regards to a differential equation for determining stress, and, as a result, internal stress is derived as an analytical solution which is expressed as a mathematical expression merely to the extent allowed by the simplistic assumption. As such, similarly to Patent Literature 2 and Non-Patent Literature 2, Non-Patent Literature 5 fails to achieve a realistic, quantitative examination which takes into account the entrainment and incorporation of air between layers of film.